Targets of the present invention can be used in the so-called isotope mass separation on-line (ISOL) technique. ISOL was invented in Copenhagen more than fifty years ago. In this method, a target of a thickness of typically one to a few tens of centimeters is bombarded with a beam of high energy particles, such as protons or heavy ions, with energies of several MeV or even GeV to produce radioactive isotopes via spallation, fission or fragmentation nuclear reactions. Typically, ISOL-target materials are made of refractory compounds, such as metals, carbides or oxides, which allow to work at high temperatures, which in turn allows to decrease diffusion and desorption times. The target material is typically placed in a target container in the form of pressed pills, metal foils, liquid metal or fibers.
Usually, materials used for targets have a microstructure or typical characteristic sizes, such as grain size, fiber diameter or foil thickness on the order of 5 to 50 μm. Upon interaction with the charged particle beam, the nuclear reaction products may diffuse through the material, into the surrounding container and effuse through a transfer line to an ion source, where they are ionized by selective ion sources, such as surface, laser or plasma ion sources. The target and the ion-source can in combination be regarded as a small chemical factory for converting nuclear reaction products into a radioactive ion beam. The ions are then electrostatically accelerated to some tens of keV, mass-separated in a dipole magnet and guided to the respective experiment or application as a radioactive ion beam.
Radioactive ion beams thus generated are of significant interest in a number of fields of research including nuclear physics, atomic physics, solid state physics, materials science, astrophysics, biophysics and medicine. Further applications of refractory materials exposed to high beam fluences concern spallation neutron sources or neutrino factories, new fission reactor lines (so called “Generation IV”) and fusion reactor technology. In all of these applications, extensive irradiation damage during operation is experienced from a predetermined spectrum or different spectra of irradiating particles at a predetermined temperature. In spallation sources, usually a mixed spectrum of protons and neutrons interacts with the targets and structural materials, whereas in new fission reactor lines, a spectrum of mainly fast neutrons is produced. In fusion technology, structural materials received high neutron fluences of typically 14 MeV.
An ideal target for isotope production would have a high production cross section of the isotope of interest for the incoming beam characteristics, good diffusion and effusion properties, limited ageing and would be operable at a high temperature. The choice of the target material generally determines the achievable yields of the given isotopes. However, there are still a number of radioactive isotopes which are not accessible yet, either because it has not been possible to produce the element of interest, or because the yield is too low. Currently, only 1500 of the 3000 isotopes predicted have been experimentally produced and identified.